drm/i915/gvt: Fix guest fail to read EDID leading to black guest console issue.

It appears missing slaves on the i2c should cause 0xff to be returned
rather than 0. So, when the Windows driver tried to address a slave
at 0x40 and got 0’s back rather than 0xff’s it must have confused it.

Signed-off-by: Paul Durrant <Paul.Durrant@citrix.com>
Signed-off-by: Xu Han <xu.han@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>